A wiring harness, which can be used as a wiring component for an automobile or the like, is composed of a wire bundle prepared by bundling a plurality of insulated wires. In general, a wiring harness that is widely used is structured such that a wire bundle is wrapped with a tape having an adhesive material or with a protective sheet, or such that a wire bundle is inserted into an insulative tube so that the outer periphery thereof may be protected therewith.
Hitherto, polyvinyl chloride (PVC)-covered electric wires, which are produced by applying an insulation coating of PVC resin onto the surface of a conductor composed of copper, copper alloy, or the like, have been widely adopted as insulated wires generally used in a wiring harness because these electric wires are excellent in terms of various properties, such as flame retardant property, electrical properties, mechanical properties, flexibility, and processability. However, the coating formed of a PVC resin contains a halogen element, and consequently a halogen gas, which causes an environmental pollution, is emitted during discarding incineration of electric wires or in case of a fire. Therefore, from the viewpoint of protection of the terrestrial environment, electric wires insulated with a halogen-free covering material (halogen-free electric wires) have recently been proposed and practically used instead of the PVC-covered electric wires.
Halogen-free polymers such as polyolefin resins are used for the halogen-free covering material (insulating coating). However, since the flame retardancy of these polyolefin resins is inferior to that of the PVC resin, a flame retardant such as magnesium hydroxide is mixed with the resins in order to improve the flame retardancy. For example, Japanese Unexamined Patent Application Publication No. 2003-226792 describes a flame retardant resin composition prepared by mixing a polymer composed of a propylene/ethylene-propylene block copolymer and a reactive polymer with metal hydroxide or the like (claim 1) and an electric wire including the resin composition as a covering material (claim 3). Examples in which magnesium hydroxide is used as the metal hydroxide are also described (Tables I to IV).
Thus, halogen-free electric wire having excellent flame retardant property can be produced and a wire bundle prepared by bundling only such halogen-free electric wire are used in some cases. However, since the PVC-covered electric wire is superior to the halogen-free electric wire in terms of processability and economic efficiency, a wire bundle including both PVC-covered electric wires and halogen-free electric wires are also used for a wiring harness and the like so that the excellent features as those of the PVC-covered electric wire and the reduction in the emission quantity of a halogen-based gas can both be attained. In addition, materials containing a PVC resin as a main component are generally used as bundling materials such as a tape, a protective material sheet, and an insulating tube that are used for bundling wires or protecting the bundled wires.
However, it has been a problem that when the halogen-free electric wire and the PVC-covered electric wire are used together, or when a PVC resin is used for the bundling material, the life of the halogen-free electric wire is shortened if the halogen-free electric wire and a PVC-containing member contact each other, thereby the degradation of the covering material of the halogen-free electric wire being caused, resulting in cracks, breakage, or the like. Accordingly, it has been sought to develop a halogen-free electric wire having a covering material which does not suffer from reduction in the life thereof even if the covering material is in contact with a member containing a PVC resin. Patent Document 1: Japanese Unexamined Patent Application Publication No. 2003-226792 (claim 1, claim 3, and Tables I to IV)